Infrared spectroscopy involves two types of devices--spectrometers that generate a spectrum from infrared energy and accessories that take the infrared energy from a sample to the detector of the spectrometer. The present invention relates specifically to an accessory to deliver infrared radiant energy to a sample and to the detector of a spectrometer.
Fourier Transform--Infrared (FT-IR) spectrometers are today the spectrometers of choice for infrared spectroscopy. Whereas a dispersive spectrometer would measure one wavelength at a time, an FT-IR spectrometer measures all wavelengths at once, thus increasing the signal received by the photodetector, reducing measuring time and generally increasing efficiency. Examples of FT-IR spectrometers are well known in the art such as, for example, the Mattson Galaxy 5000. While this type of spectrometer is used in this application as an example of a suitable instrument, it is to be understood that the present invention has wide applicability beyond just FT-IR spectroscopy.
Spectroscopic analysis using radiant energy in the infrared regions of the spectrum has enormous importance independent of the particular type of spectrometer being used. Many chemical induced compounds can be identified by their distinct characteristic spectral signatures in an infrared, energy beam. Infrared spectral analysis offers a far more economical way of obtaining information than alternatives such as nuclear magnetic resonance (NMR), spectroscopy, also called magnetic resonance imaging (MRI), spectroscopy, since infrared analysis involves little more than simply shining a beam of light onto the sample. Infrared spectroscopy often works where these other, more elaborate methods do not.
Making infrared spectroscopy work, however, requires that the infrared energy first get to the sample and then get to a detector of the spectrometer. Doing this is far easier to state than to implement in practice. Infrared energy is invisible to the human eye. Selecting the sampling area therefore must be done in some indirect manner. The range of wavelengths covered by the infrared part of the spectrum is many times greater than for visible energy. There are few materials that are transmissive of infrared light and none that are equally refractive over all infrared wavelengths. Moreover, no known material can transmit all wavelengths of infrared energy equally, so any attempt to shine a beam of infrared radiant energy through a material will effectively change the wavelength distribution (color) of the beam. Accessories for infrared spectroscopy therefore usually use mirror optics since infrared lenses are impractical.
Another characteristic of infrared radiant energy is that it has a much longer wavelength than visible light. This characteristic presents a problem in that many of the things that one would want to observe using infrared energy, such as fibers, are so small that their size is comparable to that of the wavelength of the infrared energy itself. The wavelength of infrared energy defines the theoretical "diffraction limit" of what can be observed using infrared illumination. Obtaining this limit requires using optics that obtain close to the best possible resolution at infrared wavelengths.
Observing small samples using radiant energy is known as microscopy. It was perhaps natural that the first attempts to look as small things using infrared energy involved merely adopting those features of visible light microscopes that worked with infrared energy. Coates, working at the time for Perkin-Elmer, produced the first commercial infrared microscope in the late 1950's using mirror optics that, since they were composed entirely of mirrors, could be used with infrared energy without modification.
The first FT-IR spectrometers were introduced in the early 1970's. The vastly increased capabilities of FT-IR spectroscopy provoked a succession of new infrared microscopes. The second generation of microscope accessory, exemplified by the Nanospec 20-IR, adapted the original Coates design to the new generation of spectrometers. The next generation of microscope accessory, represented by the Digilab UMA 100, added the ability to analyze a microscopic area on a sample by reflecting infrared energy off its surface by splitting the aperture of a Cassegrainian mirror optic into input and output segments. An intercepting mirror was positioned as close as practical to the secondary mirror of the Cassegrainian mirror optic at a location that functioned as a "Fourier plane" to split the aperture of the microscope accessory for both reflective and infrared transmissive samples. The fourth generation of infrared microscope accessory, represented by the IR-Plan.RTM., shifted the intercepting mirror back from the secondary of the Cassegrainian mirror optic to an optically equivalent position. The resulting design required splitting the aperture of the Cassegrainian mirror optic only for reflective samples. A transmissive sample could be sampled using the full aperture of the microscope accessory.
The second, third and fourth generation microscope accessories proved to be successful and helped to establish the utility of microscopy in infrared spectroscopy. The inventor of the present application was an inventor of the third and fourth generation microscope accessories. See, U.S. Pat. Nos. 4,653,880, 4,877,960 and 4,878,747.
Their success and acceptance notwithstanding, all previous designs for a microscope accessory suffer from a fundamental problem. Starting with the first generation, the designers of infrared microscope accessories, including the applicant in his prior work, sought to duplicate the capabilities of existing visible light microscopes by merely using the components of a visible light microscope that worked with infrared energy. The problem with this approach is that infrared microscope accessories perform a substantially different function than do visible light microscopes. Merely duplicating a visible light microscope did not address these differences. Whereas a visible light microscope is designed to survey the widest possible field just for a given magnification, the purpose of an infrared microscope accessory is to sample a small, well defined area on the sample. Doing this with the same optics used for the visual survey is not necessary because the area of the sample to be measured spectroscopically is generally only a small section of the entire visual field of view. The result of merely duplicating visual light microscopes in a spectroscopic accessory that is over engineered, overpriced, and excessive when compared to what is needed for spectroscopic measuring of a part of a sample.
The over engineering of the infrared microscope accessory has also produced a significant problem when used in reflectance observations. Prior generations of microscope accessories performed spectroscopic measurements of samples that reflect radiant energy by dividing the aperture of a Cassegrainian mirror optic between input and output segments. This aperture division necessarily cut both the spatial resolution and throughput efficiency of the microscope accessory in half so that the available infrared energy never made it to the sample. Never was all the available energy directed to only a part of the Cassegrainian mirror optic, probably because the input pupil was circular and bringing a circular beam into only a part of the circular aperture would illuminate far too little of the aperture and reduce spatial resolution by far more than half.
Another problem with previous microscope accessories was that they all operate in either a visible inspection mode or an infrared sampling mode but never both. This false duality between visual survey of the sample and its spectroscopic analysis meant that the spectroscopist never really knew what was being measured when it was measured spectroscopically. Separating measuring from sampling also was unnatural since a microscopist would normally measure a sample while viewing it. The either/or choice between sampling and viewing, moreover, is not a function of Cassegrainian mirror optics; a single imaging optic, such as a Cassegrainian mirror optic, can be divided into different regions as shown by the previous aperture dividers used to observe reflective samples. See, for example, U.S. Pat. Nos. 4,653,880, 4,878,747, 4,915,502 and 5,011,243. There is no reason such aperture division could not extend to multiplexing visual surveying and infrared sampling provided that known steps are taken to filter out any He-Ne-laser light; it simply was not done.
There is a need in the art of FT-IR microscopy for a microscope accessory that will provide a wide field image for use in surveying a sample and a narrow field just for use in spectroscopically measuring a selected area of the sample. There is also a need to increase the spatial resolution and throughput efficiency of a microscope accessory when it is used to observe a reflecting sample. There is also a need for a microscope accessory capable of multiplexing the accessory to accommodate surveying and sampling simultaneously. None of these needs have been met in the first four generations of infrared microscope accessories discussed above.
It is an objective of the present invention to advance to a fifth generation of infrared microscope accessory that overcomes some or all of the disadvantages of previous microscope accessories. Another objective is a microscope accessory with a wide field of view for surveying a sample and a narrow field of view for measuring a selected area of that sample. Another objective is a microscope accessory that operates with substantially full spatial resolution and throughput efficiency when sampling reflective samples just as for transmissive samples. Another objective is a microscope accessory that can survey and sample simultaneously. It is also an objective of the present invention to produce a microscope accessory that is inexpensive to manufacture but that still has all the capabilities of larger, more expensive and over engineered microscope accessories.
The present invention attains these and other objectives with identical symmetrical aberration canceling optics having unitary magnification that image of a narrow field of view for making spectroscopic measurements and a separate viewing system that simultaneously provides a wide field of view of the entire sample image plane. The identical symmetrical aberration canceling mirror optics can comprise parabolic mirrors and the visual observation system can be supplied by sacrificing part of one mirror through which to view the sample image plane. The measuring and viewing systems allocate the sample aperture available to the microscope accessory, defined herein as the 2T sterradians of solid angle surrounding each side of the sample image plane, between and among the functions to be performed by the microscope accessory. Spectroscopic measurements can take place at the same time as visual observations since the separate functions occupy separate segments of the sample aperture. The identical symmetrical aberration canceling mirror optics can have a high numerical aperture to accommodate a larger portion of available sample aperture. This high numerical aperture gives the microscope accessory a measurement system with plenty of aperture to allocate between incident and reflected energy. The measurement system can measure a reflective sample without any loss of efficiency and with spatial resolution comparable to or greater than that attained with Cassegrainian mirror optics.
The present invention further facilitates confocal infrared microscopy. Masks can be positioned at remote images of the remote plane to select the area of the sample to be measured spectroscopically with the measurement system. A mirror can be positioned at one field stop to facilitate more precise sampling of transmissive samples.
These and other features of the present invention have the advantage that the high numerical aperture of the unitary focusing objectives provide diffraction-limited performance over the area needed for spectroscopic measurements and only that measuring area. The present invention actually improves resolution by increasing the numerical aperture of the measurement system using less expensive, narrow field imaging optics such as identical symmetrical parabolic mirrors rather than more expensive, wide field Cassegrainian mirror optics. The large numerical aperture allows aperture to be viewed for what it is, the full solid angle about the sample, rather than as something defined and limited by a single imaging optic. The enlarged concept of aperture provides plenty of room for multiplexing the observational and spectroscopic functions without any sacrifice in throughput efficiency. Another increase in efficiency comes from enabling viewing and sampling to happen simultaneously rather than sequentially.
All these advantages, and more, are obtained with the microscope accessory disclosed in detail below.